Compensating base for lead-frame bonding

ABSTRACT

Bonding of lead frames to ceramic substrates is performed on a bonder with a compensating base. The compensating base includes a series of cantilever spring elements attached to a rigid support. The elements are reduced in cross-sectional area near the point of attachment to the rigid support. A free end of each of the spring elements is placed under each point of the substrate to which a lead is to be bonded. The elements accommodate to variations in thickness and waviness of the substrate so that it does not crack during the bonding operation.

United States Patent Piechocki, deceased et a1. Reissued Aug. 12, 1975[5 COMPENSATING BASE FOR LEAD-FRAME 3,132,239 5/1964 I 'schollhammer219/161 X BONDING 3,483,067 12/1969 Staats eta]. 156/580 3,541,673 111970 Cushman 269 310 x [75] Inventors: Ben mm le kl, deceased, late3,608,809 9/1971 Cushman 228/4 x of Bethlehem, Pa.,'by ,Stephen H.3,669,806 6/1972 -Cushman 156/580 Fletcher.v Short Hills, NJ. 1

[73] Assignee: Western Electric Company,

Incorporated New York Primary Examiner- Francis S. Husar [22] Filed:Jan. 28, 1974 Assistant Examiner' Robert J. Craig A t F' Y. 1 pp NO:437,079 Attorney, gen 01: zrm R Peters Related US. Patent Documents vReissue of: 57 ABSTRACT [64] Patent No.: 3,729,810

Issufidi y 1973 Bonding of lead frames to ceramic substrates is perpp207,854 formed on a bonder with a compensating base. The Filedi 1971compensating base; includes a series of cantilever 7 spring elementsattached to a rigid support. The elel l US; 8 9/ ments are reduced "incross-sectional area near the 269/3 228/233 point of attachment 't'o therigid support. A free end of [5 l Int. Cl 823k 5/22 each of the springelements is placed under each point 1 Field Of Search of the substrateto which a lead is to be bonded. The 269/37, 296, 310 elementsaccommodate to variations in thickness and waviness of the substrate sothat it does not crack dur- [56] R f r n s C t ing the bondingoperation.

UNITED STATES PATENTS 2,727,650 12/1955 Maynihau et a1. 29/493 4 Claims,3 Drawing Figures 2 Sheets-Sheet 1 Reissued Aug. 12, 1975 Re. 28,509

2 Sheets-sheaf. 2

Fig-2 COMPENSATING BA FOR LEAD-FRAME I 'B QN )IN G Matter enclosed inheavy brackets I appears in the original patent but forms no part ofthis reissue specification; matter printed in italics indicates-theadditions made by reissue. I

3,608,809 issued to R. H. Cushman on Sept. 28, 1971.)

Ceramic materials ,used as substratesfor thin-film circuits arenotoriously wavy and non-.uniform in thickness. Thermocompressionbonding of. lead frames to the circuits on such ceramic substratesproduces highly localized forces and stresses. The results of thesestresses on the non-uniformly shaped substrates is a certain degree ofirreparable cracking of the substrates with a consequent loss of entirethin-film circuits.

The difficulties of providing compensation are compounded by the factthat the thin-film circuits have'very closely spaced leads. It isdesirable to independently support each lead position during bonding.Such independent support is very difficult to achieve because the spaceavailable for spring elements or the like isextremely limited. r lVarious systems have been proposed and used providing the desiredcompensation during bonding. While a number of the systems have appearedto be successful. no one of the systems has been totally successful ineliminating cracking of ceramic-substrates during bonding.

to r

circumstances 7 where closely-spaced leads are being bonded. I I

These and other objectives are achieved by providing abonder with acompensating base that includes a plurality of cantilever springelements extending from a rigid support and corresponding in location todesired bonding points. The springs deflect to provide for stress reliefduring bonding.

BRIEF DESCRIPTION OF THE DRAWINGS will be more readily understoodfromthe following de- At low volume production. a'certain amount of crackingis economically tolerable. However, when produetion volume reaches highlevels. a small-percentage of breakage results in large absolute numbersof discarded thin-film circuits. The cost of such-damage becomessignificant and highly undesirable.

in addition to providing compensating arrangements to prevent crackingof substrates. attempts have been made to perform bonding operations ata slow speed. Also bonding has been performed byoperating on only onelead at a time. These slow bonding processes. however. have the obviousdisadvantage that high volume production can only bc achieved byemploying a great number of machines and a great numbcr'of employees tooperate the machines [Cle,arly. such schemes for preventing cracking areeconomically unattractive.

SUMMARY OF THE INVENTION It is therefore anobject of this invention toprovide an improved compensating system for a thermocomprcssion bonderwhich will substantially reduce the frequency of cracking of brittlesubstrates.

It is a further object of the invention to provide a compensating systemfor a bonder which will permit crack-free operation at high bondingspeeds.

It is a still further object of the invention to provide a compensatingsystem hich will operate effectively in tailed description of specificembodiments thereof. when read in conjunction with the appended drawingsin which;

FIG. I is a perspective view of an inventive compen sating base for abonding apparatus with various portions of the base and workpiecesremoved for purposes of clarity.

FIG. 2 is an end view of the inventive compensating base of FIG. 1showing a bonding head engaged with a workpiece.

FIG. 3 is an end view of a spring element of the base of FIG. I with anexaggerated illustration of the mode of deflection of the springelement.

DETAILED DESCRIPTION Referring now to FIGS. 1 and 2, a compensatingbase..designated generally by'thc numeral 20. is shown with a thin-filmcircuit 21 formed on a substrate 22 positioned for bonding. A lead frame24 overlies the substrate 22. The base 20 is provided with a pluralityof movable pins 26 projecting through a platform member 28. The pins 26are positioned so that one pin underlies each lead of the lead frame 24.

The pins 26 are free to move vertically within holes formed in theplatform 28. The bottom end of each of the pins 26 is provided with ahead 27 which rests on a compensating spring element 30. As shown inFIGS. 2 and 3. when a bonding head or thermodc 32 is pressed against thesubstrate 22 and the leads of the lead frame 24'. the pins 26 are pusheddownwardly against the spring elements 30. Each one of the pins 26 isfree to move independently so that the proper vertical position isprovided for a particular portion of the ceramic substrate 22. Since thesubstrates 22 are not necessarily planar or uniform in thickness. eachof the individual pins 26 may assume a vertical position which isdifferent from that of the other pins.

A plurality ofthe spring elements 30 are illustratively shown as beingintegral with one of a pair of singlepiece compensating units 34. It canbe seen that each of the units 34 have been produced by machining a slot35 along a horizontal axis of the unit. The slot 35. of course does notextend through the entire depth ofthe unit 34. The spring elements 30are separated from one another by machining a series of parallelseparating slots 36 along the length of each of the units 34.

At the closed end of the slot 35 a hole 40 has been drilled along theentire length of the unit 34. The resultant structure of the unit 34 isone in which the spring elements 30 can move independently of oneanother in a cantilever fashion but are attached to a rigid base 37. Inone advantageous arrangement shown in the drau ings. the point ofattachment of the spring elements 30 to the rigid portions of the unit34 has a reduced cross sectional area due to the presence of the hole40. Virtually all of the deflection of the spring elements 30 takesplace in these reduced cross-section regions. The portions of the springelements 30 having the larger crosssectional areas are substantiallyrigid. This contiguration and resultant deflection pattern is shown inFIG. 3. The result of this type of shaping and deflection system is thatthe force needed to deflect the free ends of the spring elements 30 islinearly related to the amount of deflection which occurs. In otherwords. a constant force-to-deflection ration exists. The configurationof the spring elements 30 permits rapid bonding to take place withoutcracking of the circuits 22.

As shown in FIG. 2. each of the pins 26 is provided with a head 27. Whenthe units 34 are assembled with the platform member 28, the springelements 30 are deflected from their relaxed position by a distanceequivalent to the projecting height of the heads 27. The pins 26 andtheir heads 27 are ground after assembly to provide an accuratelydetermined displacement. This displacement places the spring elements 30in a prestressed state. The pre-stressed state is a desirable one.because the neutral or return position of the free ends of the pins 26is always positively determined.

A stop member 43 is provided to limit the vertical travel of the freeends of the spring elements 30. The member 43 has a thickness such thatthe distance between the lower side of the spring elements 30 and theupper side of the member 43 is limited. The distance is limited to theextent that when the spring elements 30 are driven into contact with thestop member 43. the attachment regions of the spring elements to theunit 34 are still in a state of elastic deformation. in other words. thecompensating unit 34 and stop member 43 are constructed so that thespring elements 30 cannot be over-stressed to the point of plasticdeformation.

While the compensating units 34 have been described as being integralstructures. it should be clear that compensating units can be easilyconstructed as a stacked arrangement (not shown) of members that havethe general configuration of one end of the unit 34. Spacers can beplaced between the stacked members to provide separation between thespring elements.

The structure of the compensating base 20 is particularly advantageouswhere a great many leads are attached to a substrate with a closelead-to-lead spacing. It has been found entirely practical to bond leadsonto substrates with a center-to-center spacing of the leads of only0.075 inch. The spring elements 30 are made 0.055 inch thick and arespaced from each other by a distance of 0.020 inch.

By way of example. it has been determined that a spring element having amoment arm of 0.300 inch and a cross-secti0nal dimension of 0.055 inch0.l25 inch performs quite satisfactorily in high speed bonding ofceramic substrates formed of alumina having a thickness of 0.027 inch.The cross-sectional area of the spring element is reduced to 0.055 inchX0.080 inch at the point of connection with the rigid base member. A verydesirable material for the spring element is Alloy 25-CAl72.H TemperBeryllium Copper available from Brush Beryllium Co. of Cleveland Ohio.

Although certain embodiments of the invention have been shown in thedrawings and described in the speci- 4 lication. it is to be understoodthat the invention is not limited thereto. is capable of modificationand can be arranged without departing from the spirit and scope of theinvention.

What is claimed is:

l. A method of bonding a plurality of leads to a brittle planar articlewhich comprises the steps of:

supporting the article with a plurality of cantilever spring elements(.\t(H(/lI1gjt'OHt a bttsc with a spucc bctnccn thc elements and thcbusc. each element being located below a point where a lead is desiredand each element being movable independently of the others; and

comprcssively engaging all otthe desired leads simultaneously againstsaid brittle supported article to bond said leads to said article.whereby the spring elements are deflected independently of one anotherinto the space l7(ll\'('(ll the clcnuntx and the base to reduce stressesintroduced to said article during said compressive engagement thuspreventing cracking of the article.

2. A method of bonding a plurality of leads to a brittle planar articlewhich comprises the steps of:

supporting the article with a plurality of cantilever spring elementsattending and s mccdjrom. and integral with. 11 common lJtlSl. eachelement being located below a point where a lead is desired and eachelement being movable independently of the others;

compressively engaging all of the desired leads simultaneously againstsaid brittle supported articles to bond said leads to said article; and

compensating for irregularity of shape of the brittle article bydeflecting the spring elements into the spacc between the elements andthc bus-c. during said compressive engagement to reduce stresses andprevent cracking of the article.

3. A method of bonding a plurality of leadsto a brittle plunur articlewhich comprixcs the steps of:

supporting the articlc on u plurality ofpinx slitlctbly extendingthrough tl platform. each pin having ufrcc cnd located below 1! pointwhere it lead is to bc bondcd and each pin having (I head oppositeitsfrcc cud to prcvcnt it from passing completely through the platform.cuch head being supported by u cuntilcvcr spring element so that eachpin is movable through the platform indepcmlcntly of the others.('tlt'll spring clcmcnt extending from (I lHISt' with u spucc betweeneach element and the base;

contprcxsivcly engaging all the leads to be bonded xi multuncouslyuguinxt .s'uid supported article to bond said leads to said article: and

conipcnsutingjor irrcgulurity ol's'hupe of the brittle urticlc by.i'liduhly moving the pins through the platform and tl(f/ll.('llll t,'the spring elements thereunder into the .rpucc bctwccn the clcnu'nts andthe base during mid comprcsxivc cngugctncnt to reduce stress and toprcvcnt cracking oft/1c article.

4. The method ol'cluim 3. whcrcinthc spring clcnu'nts are provided withon initial deflection by their heads being positioncd bt'tivccn theplatform and such clcnu'nts .vo that thc pins ussuntc ttn initiulprcdctcrmincd m'utrnl position.

1. A method of bonding a plurality of leads to a brittle planar articlewhich comprises the steps of: supporting the article with a plurality ofcantilever spring elements extending from a base with a space betweenthe elements and the base, each element being located below a pointwhere a lead is desired and each element being movable independently ofthe others; and compressively engaging all of the desired leadssimultaneously against said brittle supported article to bond said leadsto said article, whereby the spring elements are deflected independentlyof one another into the space between the elements and the base toreduce stresses introduced to said article during said compressiveengagement thus preventing cracking of the article.
 2. A method ofbonding a plurality of leads to a brittle planar article whiCh comprisesthe steps of: supporting the article with a plurality of cantileverspring elements extending and spaced from, and integral with, a commonbase, each element being located below a point where a lead is desiredand each element being movable independently of the others;compressively engaging all of the desired leads simultaneously againstsaid brittle supported articles to bond said leads to said article; andcompensating for irregularity of shape of the brittle article bydeflecting the spring elements into the space between the elements andthe base, during said compressive engagement to reduce stresses andprevent cracking of the article.
 3. A method of bonding a plurality ofleads to a brittle planar article which comprises the steps of:supporting the article on a plurality of pins slidably extending througha platform, each pin having a free end located below a point where alead is to be bonded and each pin having a head opposite its free end toprevent it from passing completely through the platform, each head beingsupported by a cantilever spring element so that each pin is movablethrough the platform independently of the others, each spring elementextending from a base with a space between each element and the base;compressively engaging all the leads to be bonded simultaneously againstsaid supported article to bond said leads to said article; andcompensating for irregularity of shape of the brittle article byslidably moving the pins through the platform and deflecting the springelements thereunder into the space between the elements and the baseduring said compressive engagement to reduce stress and to preventcracking of the article.
 4. The method of claim 3, wherein the springelements are provided with an initial deflection by their heads beingpositioned between the platform and such elements so that the pinsassume an initial predetermined neutral position.